The decades since the advent of xerographic-type printing or copying have witnessed improvements to the quality of electrostatographic reproduction. Much of this improvement has concerned the recognition of problems associated with existing structure and their effective solution.
One such problem is extant in xerograhic printers of the type using a magnet roll type developing unit. In such a unit, a rotating sleeve within the developer housing revolves about a series of stationary magnets. The interplay of developer in the sump and lines of magnetic force causes developer to form a brush-like structure about the sleeve. The developer brush is used to form an image on a photoreceptive belt. While this is an effective approach to electrostatographic reproduction, it is not without problems. One such problem is the tendency of fine toner particles to be blown out from the developer housing into the remainder of the reproduction machine. The fine size of these particles makes them an invasive presence in an environment where such contamination can degrade the performance of the machine.
The immediate causative force behind this dispersal of toner particles from the developer housing is aerodynamic. The magnet roll developing unit creates a brush-like structure comprising whiskers of developer on the surface of a magnet roll sleeve rotating about the magnet roll. As the whiskers rotate with the sleeve and coact with the developer housing, they can act as a vane pump, creating regions of high and low pressure within the developer housing. The high pressure regions act to propel toner through small holes and gaps in the developer housing into the remainder of the reproduction machine.
Others in the field have attempted to solve this problem. U.S. Pat. No. 4,377,334 to Nishikawa shows the use of vent structure to reduce internal air pressure. U.S. Pat. No. 4,583,112 to Morano et al. shows the use of filtration structure to prevent toner particles from escaping into the machine. U.S. Pat. No. 4,800,411 to Tanaka et al. uses an air flow passage in a toner fiber limiting plate located within the developer housing to create localized air flows intended to reduce pressure differentials. The Tanaka patent is directed towards pressure differentials set up by counter-rotating magnet roll sleeve and developer drum. The pressure variations that it seeks to address are localized, involving adjacent regions of high and low pressure situated in the upper half of the developer unit.
There remains a need for a simple means for modifying the structure of a developer housing so as to prevent the creation of harmful air pressure differentials in general and air pressure differentials that arise across widely displaced regions within a developer housing.